1. Field of the Invention
The present invention relates to imaging technology, and more particularly, to a technique for accessing pixels in an active pixel sensor array. Embodiments of the invention may be used, for example, in cameras and in applications such as star tracking for satellite attitude control.
2. Description of the Related Art
A new image sensor technology is being developed as a successor to charge coupled device (CCD) technology. This new technology utilizes complementary metal oxide processing (CMOS) to construct an active pixel sensor (APS) that is responsive to light input over its field of view. An APS is an array of photosensitive elements known as pixels, each pixel possessing simple electrometers capable of converting photoelectric charge to an electrical current that can readily be measured. Each of the pixels can be accessed individually through various addressing schemes. This technology is contrasted to that of CCD technology where each photosensitive element transfers it's charge to the next element in a “bucket brigade” fashion until the charge reaches a common electrometer for the entire CCD that is capable of converting photoelectric charge to an electrical current.
Star tracking in the past has been accomplished by electronic imaging, typically using CCDs. However, future celestial tracking space navigation methods impose challenging requirements. Future host spacecraft should be smaller, have less mass, and require less power than present host spacecraft. Therefore, a list of features desired for future star tracking navigation devices includes miniaturization, small mass, and low power consumption. Radiation hardness, high fill factor, high sampling resolution, windowing (random access to pixels within a frame and programmable integration within windows), wide dynamic range (programmable integration time), and real time pixel access are also desirable in an imaging sensor.
U.S. patent application Publication No. U.S. 2001/0040631 A1 to Ewedemi et al. discloses a sensor array with a memory interface. Pixel data from the sensor array is stored in a local memory. An interface is coupled to the local memory and accesses the local memory for exporting the pixel data. The interface does not access the sensor array. The data is most current at a time of transfer of the data from the sensor array to the local memory, and the data ages after the transfer. Consequently, the pixel data may not be sufficiently current for some real-time processing applications.